Arrangement and method of determining properties of a surface and subsurface structures

ABSTRACT

An arrangement for determining four-dimensional properties of an interface of an object, including a light source includes: a unit for forming photonic jets, a unit for performing large field of view interferometric imaging of the interface and their combination, a unit for passing the light being close to the interface and direct the light to the interface, and an image unit. The arrangement includes a unit for performing phase shifting interferometric imaging of the interface, imaging a unit for receiving light from the interface modulated by e.g. microspheres for forming super-resolution image information by combining light interferometry with the photonic jets, and a processor unit for determining four-dimensional properties of the interface on the basis of the image information formed by the phase shifting interferometric imaging by utilizing effect of the photonic jets. The arrangement also can also include a unit to carry out the measurement using polarized light.

FIELD OF THE INVENTION

Low coherence interferometry (LCI), particularly Scanning White LightInterferometry (SWLI), is a widely used 3D surface characterizationmethod featuring sub-nanometer resolution in vertical direction. Bycombing SWLI with optical jet structures one achieves 3D superresolution imaging.

STATE OF THE ART

Light sources for SWLI are halogen lamps or white-light light-emittingdiodes (LED) imaged into the objective pupil in a Köhler geometry. Theillumination field and aperture are controlled. The light source may bestroboscopic to freeze oscillating motion, and the emission spectrum maybe electronically controllable. The source wavelengths are visible orinfrared (1-2 or 10 μm).

Area imaging sensor camera (CCD, CMOS) for SWLI has from 640×480 pixelsto 40+ million pixels. Camera selection involves field size and numberof pixels as well as acquisition speed, response linearity, quantum welldepth, and digitization resolution.

Reference surface/mirror (Michelson type interferometer configuration)is aluminized glass, silicon carbide (SiC), or bare glass, depending onsample reflectivity. The mirror in a Mirau type interferometerconfiguration is a small metallic coating, slightly larger in diameterthan the field of view, on a transparent reference plate. The opticalsystem of a LCI device employs infinite conjugate optics withtelecentric imaging, and magnification determined by the combined actionof the objective and tube lens. The measurement principle requiresengineering and adjusting the objective so that the zero group-velocityoptical path difference position is identical to the position of thebest focus. The Michelson objective achieves this with a dispersionbalanced cube beam splitter. In a Mirau microscope, the beam splitterand reference plate should match in optical thickness to minimizedispersion.

Lateral resolution can be determined e.g. in the following way. The Abbediffraction limit (d_(x,y)) is the smallest lateral periodicity in astructure, which can be discriminated in its image:

d _(x,y)=1.22λ/2NA   (1)

where λ is the center wavelength of the light and NA is the numericalaperture of the lens. When imaging with visible light (λ˜400-750 nm) andcommonly used objectives with NA=1.4, the lateral resolution isapproximately 200 nm.

The diffraction limit is due to loss of evanescent waves in the farfield. These evanescent waves carry high spatial frequencysub-wavelength information of an object and decay exponentially withdistance.

The axial image resolution (d_(z)) is 2-3 times larger than the lateralresolution, around 440 nm.

d _(z)=2nλ/NA²   (2)

where n is the refractive index of the medium in which light propagates.

Any microscopy technique that overcomes the resolution limit by factorof 2 or higher is considered to provide super-resolution.

Scanning electron microscopes (SEM) can provide 3D nano-resolutionimages by e.g. using several electron guns or detectors simultaneously.These devices do not provide super resolution.

Low coherence interferometry, i.e. SWLI, overcomes the axial resolutionlimit and allows superior-resolution along the vertical direction(sub-nanometer).

Near field techniques offer lateral and vertical super-resolution.Optical near-field microscopy is based on measuring scattered light,close to a near-field probe, which is generated by optical near-fieldinteraction between the nearfield probe and a specimen. Near-field probetips of known shape are used to achieve high local resolution, e.g.contacting atomic force microscope (AFM) and noncontacting scanningtunneling microscope (STM) tips. The near-field probe can be illuminatedby focused light to generate scattered light.

There are noncontacting techniques based on photonic nanojets thatpermit 50 nm lateral resolution in the x-y plane but much worse axialresolution (z-direction).

The photonic nanojet is a narrow, high-intensity, non-evanescent lightbeam that can propagate a distance longer than the wavelength λ afteremerging from the shadow-side surface of an illuminated losslessdielectric microcylinder or microsphere of diameter larger than λ. Thenanojet's minimum beamwidth can be smaller than the classicaldiffraction limit, in fact as small as ˜λ/3 for microspheres.

US patent application 2010/0245816 Al describes near-field Ramanimaging, performed by holding a dielectric microsphere (e.g. ofpolystyrene) on or just above the sample surface in a Raman microscope.An illuminating laser beam is focused by the microsphere to producenear-field interaction with the sample. Raman scattered light at shiftedwavelengths is collected and analyzed. The microsphere may be mounted onan AFM cantilever or on some other scanning probe microscope thatprovides feedback to keep it in position relative to the sample surface.Alternatively, the microsphere may be held on the sample surface by theoptical tweezer effect of the illuminating laser beam. One disadvantageof this device is the vertical resolution which depends strongly on theconfocal design of the Raman microscope being used. For a true confocaldesign (which incorporates a fully adjustable confocal pinhole aperture)depth resolution is on the order of 1-2 μm.

Probes of scanning near-field optical microscopes create electromagneticfield characteristics that are maximally localized near a nano-sizedpoint (miniature apertures and tips, fluorescent nano-particles andmolecules, dielectric and metal corners). However, the probe field,which is distributed across a larger area, can provide super-resolutionas well. For this purpose, the field spectrum should be enriched withhigh spatial frequencies corresponding to small dimensions of thesample. As examples of such nearfield probes, US patent 2009/0276923 A1proposes and theoretically studies models of optical fibers whoseend-face features sharp linear edges and randomly distributednanoparticles. These kinds of probes are mechanically more robust thanconventional probes—fabricated by using a combination of a two-stepchemical etching and focused ion beam milling and their manufacturingdoes not require nanoscale precision. The optical probes enablewaveguiding of light to and from the sample with marginal losses bydistributing and utilizing the incident light more completely thanconventional probes. Numerical modeling shows that, even withsubstantial measurement noise, these probes can resolve objects that aresignificantly smaller than the probe size and, in certain cases, canperform better than conventional nanoprobes. One disadvantage of thisdevice is that it measures point by point.

Patent application document WO 2013/043818 Al describes a system andmethod for imaging a surface, including a nano-positioning deviceincluding a cantilever with an optically transparent microsphere lenscoupled to the distal end of the cantilever. An optical component canfocus light on at least a portion of the surface through the microspherelens, and the focused light, if any, reflects back from the surfacethrough the microsphere lens. A control unit communicatively coupledwith the nanopositioning device can be configured to position themicrosphere lens at a predetermined distance above the surface. Onedisadvantage of this device is the vertical resolution which isdiffracted limited.

In far-field microscopy, imaging contrast is often low andunsatisfactory due to out-of-focus light in the final image. To enhancecontrast, one can optimize the microscope lighting condition and imagingsoftware settings during imaging. In contrast to far-field microscopy,confocal microscopy techniques generally have better optical contrastand improved resolution; this is achieved by placing a tiny pinholebefore the detector to eliminate out-of-focus light in the final image.When combining laser confocal microscopy with micro-spheres, multipleconcentric rings in the confocal imaging appears if one uses closelypositioned spheres. These rings result from near-field interactionsbetween the particle or spheres and the substrate under coherent laserillumination. In contrast, an incoherent light source, renders thisissue less obvious in far-field microscopy. These rings degrade imagingquality, which may pose a practical limit on the minimum feature thatcan be resolved in confocal imaging.

Prior art embodiments suffer from these artefacts that might wrongly beinterpreted as objects in the image. For isolated and known particles,one can still see the true image of the objects through the particles.The artefact issue is less obvious in a far-field nanoscopy system wherean incoherent lighting source is often used.

Prior art describes polarization as a way to enhance contrast especiallyin bio-imaging. There are many studies on polarization in far-fieldmicroscopy and also several studies on polarization-SWLI for bothimaging static and moving samples. There are some studies on the use ofpolarization in near field microscopy but it has never been used in 3Dsuper-resolution imaging. Prior art publications fails to present 3Dcalibration at the nanometer scale.

SHORT DESCRIPTION OF THE INVENTION

An object of the present invention is to achieve an improved 3D superresolution imaging system and method for determining surfacetopographies and/or subsurface structures. This is achieved by anarrangement for determining three-dimensional properties of an interfaceof an object. The arrangement comprises means for interferometricimaging which means comprises:

-   -   a light source,    -   imaging means for forming an interference image based on        interference between light arriving at the imaging means from        the interface of the object and light arriving at the imaging        means from a reference path related to the interferometric        imaging, and    -   means for forming the reference path from the light source to        the imaging means, for directing light from the light source        towards the interface of the object, and for directing light        from the interface of the object to the imaging means.

The arrangement further comprises means constituting a near fieldmodifying structure for forming, from the light directed towards theinterface of the object, one or more photonic jets directed to theinterface of the object, wherein the means for interferometric imagingis arranged to perform the interferometric imaging through the meansconstituting the near field modifying structure.

An arrangement according to an exemplifying embodiment of the inventionis an arrangement for determining four-dimensional properties of aninterface of an object. The arrangement comprises a light source, meansfor forming photonic jets to be utilized in imaging of the interface,means for performing large field of view interferometric imaging of theinterface and of a combination of the interface and the means forforming the photonic jets, means for passing said light being close tothe interface and direct the light to the interface, and said meanscreate an image, and the arrangement comprises means for performingphase shifting interferometric imaging of the interface, imaging meansfor receiving light from the interface modulated by at least one ofmicrospheres and near field modifying structures for formingsuper-resolution image information by combining light interferometrywith the photonic jets, and a processor unit for determiningfour-dimensional properties of the interface on the basis of the imageinformation formed by said phase shifting interferometric imaging byutilizing the effect of the photonic jets.

An object of the invention is also a method for determiningthree-dimensional properties of an interface of an object. The methodcomprises:

-   -   directing light from a light source to a reference path related        to interferometric imaging,    -   directing light from the light source towards the interface of        the object, and    -   performing the interferometric imaging so as to form an        interference image based on interference between light arriving        from the interface of the object and light arriving from the        reference path,

The above-mentioned interferometric imaging is performed through meansconstituting a near field modifying structure for forming, from thelight directed towards the interface of the object, one or more photonicjets directed to the interface of the object.

A method according to an exemplifying embodiment of the invention is amethod for determining four-dimensional properties of an interface of anobject. In the method is produced light, is formed photonic jets to beutilized in imaging of the interface, is performed large field of viewinterferometric imaging of the interface and of a combination of theinterface and the means for forming the photonic jets, is passed saidlight close to the interface and is directed the light to the interface,and is created an image, and is performed phase shifting interferometricimaging of the interface, is received light from the interface modulatedby at least one of microspheres and near field modifying structures forforming super-resolution image information by combining lightinterferometry with the photonic jets, and is determinedfour-dimensional properties of the interface on the basis of the imageinformation formed by said phase shifting interferometric imaging byutilizing the effect of the photonic jets.

The invention is based on photonic jets which are utilized in imaging ofthe interface, and on performing large field of view interferometricimaging of the interface and of a combination of the interface and themeans for forming the photonic jets. Light is passed close to theinterface and is directed to the interface, and is created an image. Theinvention can be further based on phase shifting interferometric imagingof the interface, and on light received from the interface modulated bymicrospheres for forming super-resolution image information by combininglight interferometry with the photonic jets.

A benefit of the invention is that label free, noncontact, large fieldof view and fast determination of four-dimensional properties of aninterface of an object can be achieved.

SHORT DESCRIPTION OF FIGURES

FIG. 1 presents first exemplary embodiment according to the presentinvention.

FIG. 2 presents second exemplary embodiment according to the presentinvention.

FIG. 3 presents preferred embodiment according to the present invention.

FIG. 4 presents an example of a surface imaged according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention can be achieved non-contacting largefield of view 3D super-resolution imaging by combining lightinterferometry for the z axis and photonic jet for the xy-plane. Thelight interferometry can be e.g. so called white light interferometry.The z axis imaging uses a real image, injects light into the near-fieldmodifying structure, e.g. a sphere, and extracts through the spherelight reflected from the interface. In the xy plane imaging is injectedlight into the sphere, is extracted light through the sphere and fromoutside of the sphere, and is used a virtual image of the interface.

In FIGS. 1, 2, and 3 are presented exemplary preferred embodimentsaccording to the invention, in which an arrangement for determiningfour-dimensional properties of an interface 100 of an object comprises alight source 102. Four dimensional means 3D (xyz dimensions) and timedomain. The interface 100 can be a surface of the object or a subsurfaceof the object, i.e. a so called buried surface. The arrangementcomprises means for forming photonic jets to be utilized in imaging ofthe interface 100 and means 105 a, b for performing large field of viewinterferometric imaging of the interface 100 and of a combination of theinterface and the means for forming the photonic jets. In one embodimentthe arrangement can comprise means for performing image stitching tostitch either separately or together both superstructure andsubstructure to have large field of view. The means for forming photonicjets can comprise at least one of a microsphere and micro cylinder andmicro-lense (e.g. Fresnel) and grid and cubes and metamaterials andnegative refractive index materials, as well as any near-field modifyingstructure of a specified and known shape or of an unspecified shape whenone can use a known target to extract a so called point spread function.Also the means for forming photonic jets can comprise e.g. polymer orpolymer-like material with photonic jets. The photonic jets can be e.g.nanojets or equivalents. In one embodiment the arrangement can alsocomprise means for performing the measurements using polarized light.

The arrangement according to the invention comprises means 108 forpassing said light that are close to the interface 100 and direct thelight to the interface, and create an image, and means 106 forperforming phase shifting interferometric imaging of the interface 100.The means 108 are preferably microspheres 108, which can be e.g.high-index microspheres 118 embedded partially or fully in asubstantially thin transparent host material 116. In one embodimentmeans 106 for moving the object can be used as the means 106 forperforming phase shifting interferometric imaging of the surface 100.The means 106 for moving the object can be e.g a glass micropipette 114attached to the microspheres 108 for moving the microspheres 108 andanother tip to locally actuate the surface of the object, which is e.g.cell. In another embodiment the means 106 for performing phase shiftinginterferometric imaging of the surface 100 can comprise utilization ofstroboscopic illumination.

The arrangement according to the invention further comprises imagingmeans 110 for receiving light from the interface 100 modulated bymicrospheres 108 for forming super-resolution image information bycombining light interferometry with the photonic jets, and a processorunit 112 for determining four-dimensional properties of the interface100 on the basis of the image information formed by said phase shiftinginterferometric imaging by utilizing the effect of the photonic jets.The imaging means 110 can be e.g. a CCD camera. In FIG. 4 is presentedan example of a surface 100 imaged according to the invention.

In one embodiment the arrangement can comprise means for performing samefield of view calibration on the basis of an improved nanoruler conceptwhere one has added a grid to the lowest step in order to allowsimultaneous z axis and xy axis calibration. The means can be e.g. astack of Langmuir Blodgett films on e.g. a microscope glass. The gridcan be created with e.g. short wavelength lithography.

In another embodiment the arrangement can comprise means 124 for formingcoherence function to achieve minimum main lobe width and sufficientside lobe reduction in order to remove impact of the photonic jet layerand to allow maximum resolution. The means 124 can be accomplished e.g.by using a light source with different coherence length or by using arough disc to break the coherence of the light source or by combining insuitable way several light sources.

In one further embodiment the arrangement can comprise means 126 formanaging polarization to create at least one of phase shift, transientimaging, and enhanced image contrast. The means 126 can be accomplishede.g. by placing polarizer in front of the light source and an analyzerin front of the large area detector or by using pixelated polarizers.

In some embodiment according to the invention the arrangement cancomprise means for accounting for the distortion of the surfacetopography created by the finite size shape of the photonic jet. Thesemeans can be incorporated e.g. by relying on deconvolution approachessimilar to those used to correct for the finite tip size in AFM imaging.

In the following is described more detailed features of the differentembodiments according to the present invention. LCI (SWLI) and thephotonic nanojet technology are combined to achieve 3D super-resolutionfeaturing tenths of nanometers lateral and vertical resolution. Thisshould provide voxels that are more equilateral (symmetric) and smallerthan previously achieved. The device permits label-free non-contactingimaging of both surfaces and buried structures that may be static or maymove. The full field of view techniques provides fast and simultaneousview of all points on a fairly large area. Traceability of the imagedimensions can be achieved using the nanoruler approach. The device,i.e. arrangement according to the invention can be hand held.

In one embodiment presented in FIG. 1 is used a SWLI setup with a Mirauinterference objective 105 b. The nanojet can be achieved by usingmicrosphere or micro cylinders or micro-lense or grid or cubes ormetamaterials or negative refractive index materials or nanoparticles ofa specified and known shape—spherical, hemi-spherical or other shape toproduce nanojets. In addition, a wetting layer, serving as a lubricant,could be used. Nanojet particles could be freely placed on the sample orembedded partly or entirely in the polymer material using e.g.self-assembly technics, forming single or multilayered structure. In thelatter case attention should be paid to the thickness of the layer.

In another embodiment presented in FIG. 2 is used a Linnik or Michelsonconfiguration 105 a, which allows use of different conventionalobjectives and which also permits layer thickness compensation in casepolymers are used as an embedding material. It also allows subsurfaceimaging, i.e. imaging of buried structures.

These embodiments in microscopy require control of the positioning ofthe microspheres during scanning. Two approaches to solve this problemare: (1) the microsphere is moved with a fine glass micropipetteattached to the microsphere, (2) high-index microspheres (TiO₂ orBaTiO₃) can be partly or fully embedded in a transparent host material(e.g. PMMA, PDMS), having a thickness similar to a standard coverslip,which is thin enough for the micro-lens or near-field modifyingstructure to be directly inserted into the gap between a conventionalmicroscope's objective lens and the sample. Preferred sizes of themicrospheres is e.g. 10 micrometers with refractive index of thematerial being e.g. 1.6, and magnification of the objectives used in thearrangement is e.g. 50×.

The embodiments according to the present invention can be utilized e.g.in the following applications:

I The invention can be utilized in drug development. It helpshigh-throughput screening. It helps development of personalizedtreatment cocktails at the bed side for cancer treatment. It is aphysical way of doing dissolution tests on complex drug-carryingdrug-delivery devices. With this super-resolution technique one canprecisely measure erosion of the drug delivery devices. This means thatone does not have to carry out chemical dissolution tests that can beslower and that may require more substance for the tests. Moreover, thesame approach can be used for any kind of nanochemistry-like approachwhere one either adds nanoparticles to a surface or to a construct orremove them either actively or passively.

II The invention can be utilized in tests of fibers and constructsproduced by ultrasound enhanced electrical spinning, —a way to producedrug-laden nanofibers. These fibers can be used e.g. in fiber constructswhose diameters are controlled to allow controlled release profiles.Such fibers could e.g. react to the surrounding glucose level andrelease insulin on demand.

In prior art the only way to image these nanoscale constructs is AFM orSEM, which are complex and slow.

III The invention allows one to rapidly take images of nanoparticles ofsize below one hundred nanometers. These kinds of nanoparticles can giveexisting failed drug components a second chance. It is important forquality assurance purposes to see these nanoparticles when you producethem. This cannot be done with SEM or AFM, because they are too slow.

IV According to the invention can be provided a tool for supersurfaceand subsurface bioimaging in a label free manner at nanometerresolution. Imaging using dyes as well as label free AFM imaging sufferaccording to prior art from serious problems.

V According to the invention can be provided a read-out device forsecurity applications where can be used embedded nanodots as a way toensure authenticity.

Although the invention has been presented in reference to the attachedfigures and specification, the invention is not limited to those as theinvention is subject to variations within the scope allowed for by theclaims according to different kind of applications.

1-20. (canceled)
 21. An arrangement for determining three-dimensionalproperties of an interface of an object, the arrangement comprisingmeans for interferometric imaging, wherein the means for interferometricimaging comprises: a light source, imaging means for forming aninterference image based on interference between light arriving at theimaging means from the interface of the object and light arriving at theimaging means from a reference path related to the interferometricimaging, and means for forming the reference path from the light sourceto the imaging means, for directing light from the light source towardsthe interface of the object, and for directing light from the interfaceof the object to the imaging means, wherein the arrangement furthercomprises means constituting a near field modifying structure forforming, from the light directed towards the interface of the object,one or more photonic jets directed to the interface of the object,wherein the means for interferometric imaging is arranged to perform theinterferometric imaging through the means constituting the near fieldmodifying structure.
 22. An arrangement according to claim 21, whereinthe arrangement comprises means for changing a phase-shift between thelight arriving at the imaging means from the reference path and thelight arriving at the imaging means from the interface of the object.23. An arrangement according to claim 22, wherein the means for changingthe phase-shift comprise means for moving the object.
 24. An arrangementaccording to claim 21, wherein the arrangement comprises a processorunit for controlling the means for interferometric imaging to produce atemporal sequence of interference images of the interface of the object.25. An arrangement according to claim 21, wherein the means forinterferometric imaging comprises means for performing theinterferometric imaging with stroboscopic illumination.
 26. Anarrangement according to claim 21, wherein the arrangement comprisesmeans for performing image stitching to stitch interference imagescorresponding to fields of view into a combined interference imagecorresponding to a combination of the fields of view.
 27. An arrangementaccording to claim 21, wherein the arrangement comprises means forperforming calibration of the interferometric imaging on the basis of ananoruler concept in which a grid is in a same field of view togetherwith the object being imaged in order to allow simultaneous calibrationin vertical and lateral directions.
 28. An arrangement according toclaim 21, wherein the arrangement comprises means for optimizing acoherence function of the light used in the interferometric imaging inorder to maximize resolution, the means for optimizing the coherencefunction comprising one of the following: the light source with acoherence length selected to provide the coherence function, a disc forbreaking coherence of the light used in the interferometric imaging, thelight source constituted by a combination of several light sources. 29.An arrangement according to claim 21, wherein the arrangement comprisesmeans for managing polarization of the light used in the interferometricimaging.
 30. An arrangement according to claim 21, wherein thearrangement comprises means for accounting for the distortion of thesurface topography created by the finite size shape of the one or morephotonic jets.
 31. An arrangement according to claim 21, wherein themeans constituting the near field modifying structure comprises one ormore particles each being one of the following: a microsphere, amicrohemisphere, a microcylinder, a microlens, a microcube, a piece ofmetamaterial, a piece negative refractive index material.
 32. A methodfor determining three-dimensional properties of an interface of anobject, the method comprising: directing light from a light source to areference path related to interferometric imaging, directing light fromthe light source towards the interface of the object, and performing theinterferometric imaging so as to form an interference image based oninterference between light arriving from the interface of the object andlight arriving from the reference path, wherein the interferometricimaging is performed through means constituting a near field modifyingstructure for forming, from the light directed towards the interface ofthe object, one or more photonic jets directed to the interface of theobject.
 33. A method according to claim 32, wherein the interface of theobject is a surface of the object.
 34. A method according to claim 32,wherein the interface of the object is a subsurface of the object.
 35. Amethod according to claim 32, wherein the method comprises changing aphase-shift between the light arriving from the reference path and thelight arriving from the interface of the object.
 36. A method accordingto claim 35, wherein the method comprises moving the object so as tochange the phase-shift.
 37. A method according to claim 32, whereinstroboscopic illumination is used in the interferometric imaging.
 38. Amethod according to claim 32, wherein the method comprises stitchinginterference images corresponding to fields of view into a combinedinterference image corresponding to a combination of the fields of view.39. A method according to claim 32, wherein the method comprisesperforming calibration of the interferometric imaging on the basis of ananoruler concept in which a grid is in a same field of view togetherwith the object being imaged in order to allow simultaneous calibrationin vertical and lateral directions.
 40. A method according to claim 32,wherein the method comprises optimizing a coherence function of thelight used in the interferometric imaging in order to maximizeresolution, the optimizing the coherence function comprising one of thefollowing: selecting a coherence length of the light source to providethe coherence function, using a disc for breaking coherence of the lightused in the interferometric imaging, using a combination of severallight sources as the light source.
 41. A method according to claim 32,wherein the method comprises managing polarization of the light used inthe interferometric imaging.
 42. A method according to claim 32, whereinthe method comprises accounting for the distortion of the surfacetopography created by the finite size shape of the one or more photonicjets.
 43. A method according to claim 32, wherein the means constitutingthe near field modifying structure comprises one or more particles eachbeing one of the following: a microsphere, a microhemisphere, amicrocylinder, a microlens, a microcube, a piece of metamaterial, apiece negative refractive index material.
 44. A method according toclaim 32, wherein the method comprises producing a temporal sequence ofinterference images of the interface of the object.